Can kelp help mitigate against ocean acidification?

— Written by Spencer Johnson

Here in Southern California, the most prominent and productive marine habitats are the underwater forests of Macrocystis pyrifera, or giant kelp. One of the ways kelp affects seawater is that it slows currents. Like a group of trees providing a wind shelter on an open hillside, kelp creates a lot of drag. As a photosynthetic organism, kelp also removes carbon dioxide from the water and releases oxygen. These effects can work together: the more that currents are slowed, the longer a given parcel of water stays in the kelp forest, allowing more carbon dioxide to be removed. Increased carbon dioxide is the root cause of ocean acidification; kelp forests may act as refuges in an increasingly acidic ocean.

The Santa Barbara Channel is a long-term ecological research (LTER) site, so a wide variety of data have been collected and compiled in it over the last few decades. A number of locations within the channel have been designated as marine protected areas (MPAs), which preserve thriving kelp forests. We have data for currents from moorings called acoustic doppler current profilers (ADCPs), and we have data for kelp biomass derived from mathematical calculations on satellite image pixels. We also have data for dissolved oxygen, which can be used as a proxy for pH. My goal for the first part of my project is to analyze these data and solidify the relationship between kelp biomass and pH. Ultimately, we want to be able to quantify the positive effects of MPAs for use in ecosystem models. However, the data collection sites don’t line up cleanly with MPA boundaries, so I’m planning to compare data inside and outside a healthy kelp forest. One site, Arroyo Quemado, got new ADCP moorings last year inside and just outside the kelp forest, augmenting the long-term mooring outside the kelp forest. These 3 moorings give me a convenient comparison to explore.

Figure 1: Screenshot of some of my initial work: From left to right, a graph of water level and tidal currents at Arroyo Quemado, tidal component data for Arroyo Quemado, and a 3-D graph of East current velocity versus North current velocity versus height above the bottom

The second part of my project will involve going out into the field and collecting some targeted physical data to expand on my findings from past data. The details for this part of the project are still very much up in the air. So far, my work has mostly involved learning how to use Matlab, a programming language that’s good for complex calculations and attractive graphs, such as the ones shown. With the help of some prewritten code and a function that runs harmonic analysis to determine tidal components, I’ve analyzed the tides for a couple LTER sites and for NOAA’s Santa Barbara data. Harmonic analysis allows me to determine the strengths of the components that go into tidal currents (most are related to the sun and moon). In doing this, I can compare current strengths and figure out how much they are slowed by kelp forests. My work with currents will help me determine a rate constant for the biological activity that controls dissolved oxygen changes. I’m excited to get further into my analysis, and to eventually go out into the kelp forest and collect some new data!

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